Space-based circuitry has required radiation hardening due to the interference of cosmic rays and solar radiation with the pn junctions of the device. Cosmic rays are composed 90% of protons and 9% alpha particles. Cosmic rays can have an energy range from 0.3×109 eV to 3.0×1020 eV. The solar wind is composed 95% of protons and electrons travelling at 400-650 km/sec. Solar flares, or coronal mass ejections (CMEs), can impact circuits with even higher energy protons and heavy ions by causing the collection of charge in p-n junctions.
High-energy protons and heavy ions lose energy in materials mainly through ionization processes. When this occurs, they leave a dense trail of electron-hole pairs as they pass through a p-n junction. Some of the resulting charge will recombine, and some will be collected at the junction contacts. In addition to the charge provided by depleted junction, indirect charge can be added from regions outside the junction due to charge funneling and diffusion. This accumulation of charge results in a very short duration pulse of current at the internal circuit node which is struck by the particle. The magnitude of the charge depends on the energy, type, and charge of the ion, the path length, and the size and sensitivity of the circuit to small current impulses, including the voltage required to switch states.
Most of the charge deposited at the circuit node occurs in time periods of about 200 picoseconds (ps), and this is known as prompt charge. There is also delayed charge as a result of diffusion which occurs in time periods of about 1 microsecond (μs) or longer. This delayed charge is the cause of dynamic memory upset and latchup. Transient effects, such as single-event upset (SEU) and multiple-bit upset (MBU) change the state of internal storage elements, but can be reset to normal operation by a simple series of electrical operations or reinitialization. Catastrophic events, such as single-event latchup (SEL) and snapback, are capable of causing physical destruction of the circuit node unless they are corrected for within a short time after they occur.
Single-event upsets that occur in storage elements are capable of changing the state of the circuit, and stored information is lost. However, the circuit still functions normally, and it can be restored to its original operating state by rewriting or reinitializing the circuit. More catastrophic damage such as latchup, where a transient accidentally creates a low resistance short-circuit parasitic path across a circuit, can lead to permanent heat damage to the silicon and can cause a chain reaction of latchup in nearby circuits. Although earth-bound CMOS circuit designs use special guardbands and clamp circuits at input/output terminals to prevent latchup, in a radiation environment like space the heavy ion or proton strikes generate enough current to trigger latchup in internal region of a CMOS device, as well as in I/O circuitry. Historically, the only way to save a circuit from latchup was to sense the excess current and immediately power down and restart the system.
As circuits continue to shrink in size and power requirements decrease, susceptibility to SEUs and to latchup is increased.